The present invention relates to a cleaning device that uses a brush roller to remove toner attached to a transfer sheet conveyance belt.
In an image forming apparatus using electrophotographic process, a toner image is formed on an image carrier (hereinafter also referred to as “photoreceptor”) and this toner image is transferred onto a transfer material (hereinafter also referred to as “transfer sheet”). After that, the transfer sheet carrying the toner image is subjected to a process of fixing by a fixing device. After fixing, the transfer sheet with the image formed thereon is ejected from the image forming apparatus. In this process of transfer, the transfer sheet is conveyed by the transfer sheet conveyance belt and is fed through the transfer section made up of a transfer member arranged to face the photoreceptor through the transfer sheet conveyance belt, whereby the toner image formed on the photoreceptor is transferred onto the transfer sheet.
The transfer sheet conveyance belt is not contaminated by unwanted toner particles since the toner image is hardly formed on the surface thereof in general cases. However, for the portion of the image such as an image control patch to be created on a position of the photoreceptor between transfer sheets and a black band to assist photoreceptor cleaning, even if the transfer member is placed in the non-transfer mode, the toner particles on the photoreceptor may be transferred onto the transfer sheet conveyance belt in some cases, since the photoreceptor is kept in contact with the transfer sheet conveyance belt. Further, if the photoreceptor contains fogged toner particles, the toner particles are transferred onto the transfer sheet conveyance belt in some cases.
The aforementioned unwanted toner particles deposited on the transfer sheet conveyance belt are transferred onto the rear surface of the next transfer sheet to cause contamination in some cases. This makes it essential to clean the transfer sheet conveyance belt for removing the unwanted toner particles deposited on the transfer sheet conveyance belt.
A blade method and a fur brush method wherein a fur brush is used as a brush roll is generally used for the cleaning. In the blade method, an elastic blade such as a rubber blade against the transfer sheet conveyance belt and the toner particles are scraped off mechanically for cleaning. In the fur blush method, a cylindrical substrate with a fur brush attached on the surface is rotated to contact the toner particles on the transfer sheet conveyance belt, and at the same time, the voltage with a polarity reverse to that of toner particles is applied to the brush roll, whereby toner particles are scraped off by electrostatic attraction, and cleaning is performed. The blade method has a problem with durability resulting from turning up of the blade or others. Thus, the fur brush method characterized by a smaller contact load is preferably used.
However, toner particles deposited on the transfer sheet conveyance belt are polarized; for example, they are negatively charged as a whole. Each toner particle, however, may be different in the electric charge amount and the toner particles of reverse polarity (positively charged) are present. FIG. 2 is a diagram representing a typical electric charge amount distribution of negatively charged toner particles. In the fur brush method, cleaning is made by electrostatically attracting toner particles by the brush roll. This makes it possible to clean the toner particles of the polarity (negative) reverse to that (positive) applied to the brush roll. However, the toner particles having the polarity (positive) reverse to that of the toner particles and having a smaller amount of electric charge pass under the brush roll without being removed by cleaning and are deposited on the rear surface of the next transfer sheet, thereby causing contamination.
For the cleaning of the transfer sheet conveyance belt, a proposal has been made of an apparatus wherein a cleaning electric field is formed between the conductive cleaning member arranged in contact with the surface of the belt-shaped member with toner particles deposited thereon, and the conductive opposed member (urging roller) arranged to face the cleaning member through the belt-shaped member, the conductive opposed member has an elastic characteristic, and the cleaning member and elastic opposed member are pressed against each other through a belt-shaped member (for example, see Unexamined Patent Application Publication No. 2002-40897).
In the Unexamined Patent Application Publication No. 2002-40897, voltage with a polarity reverse to a charging polarity of toner particles is applied to the cleaning roller to form the cleaning electric field. This electric field ensures that the toner particles attached to the belt-shaped member are moved toward the cleaning roller member so that the surface of the belt-shaped member is cleaned. Further, the contact pressure between the cleaning member and belt-shaped member is kept constant, thereby maintaining excellent cleaning performances. This arrangement avoids cleaning failure of the transfer sheet conveying member and prevents such a trouble as contamination on the rear of the transfer sheet.
However, as described above, the toner particles having the polarity (positive) reverse to that of the toner particles and having a smaller amount of electric charge pass under the cleaning roll without being removed by cleaning, and are deposited on the rear surface of the next transfer sheet, thereby causing contamination in some case. The Unexamined Patent Application Publication No. 2002-40897 fails to consider the toner particles passing under the cleaning roller and does not mention any solution to this problem.
One aspect of the present invention is a cleaning device for cleaning a transfer sheet conveyance belt, wherein a toner image of an image carrier formed by an image forming section is transferred onto a transfer sheet while the transfer sheet is placed on and conveyed by the transfer sheet conveyance belt, the cleaning device including: a first brush roll rotating in contact with the surface of the transfer sheet conveyance belt, the first brush roll being provided with a voltage with a polarity reverse to a charging polarity of toner particles; and a second brush roll arranged downstream of the first brush roll in a moving direction of the transfer sheet conveyance belt and rotating in contact with the surface of the transfer sheet conveyance belt, the second brush roll being provided with a voltage with the same polarity as the charging polarity of the toner particles.